The present invention relates to light emitting devices and, more particularly, to semiconductor light emitting devices that include red phosphors that exhibit good color rendering properties and can achieve high luminous flux values.
Light emitting diodes (“LEDs”) are solid state lighting devices that are capable of generating light. LEDs include both semiconductor-based LEDs and organic LEDs (which are often referred to as OLEDs). Semiconductor-based LEDs generally include a plurality of semiconductor layers that may be epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
The “peak” wavelength of an LED refers to the single wavelength where the radiometric emission spectrum of the LED reaches its maximum as detected by a photo-detector. LEDs typically have a narrow wavelength distribution that is tightly centered about their “peak” wavelength. For example, the spectral power distributions of a typical LED may have a full width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width). LEDs may also be identified by their “dominant” wavelength, which is the wavelength where the radiometric emission spectrum of the LED, as perceived by the human eye, reaches its maximum value. The dominant wavelength thus differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
As most LEDs are nearly monochromatic light sources that appear to emit light having a single color, LED-based light emitting devices that include multiple LEDs that emit light of different colors have been used in order to provide solid state light emitting devices that generate white light. In these devices, the different colors of light emitted by the individual LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelength and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by surrounding a single-color LED with a luminescent material that converts some of the light emitted by the LED to light of other colors. The combination of the light emitted by the single-color LED that passes through the luminescent material along with the light of different colors that is emitted by the luminescent material may produce a white or near-white light. For example, a single blue-emitting LED chip (e.g., made of indium gallium nitride and/or gallium nitride) may be used in combination with a yellow phosphor, polymer or dye such as for example, cerium-doped yttrium aluminum garnet (which has the chemical formula Y3Al5O12:Ce, which is referred to herein as a “YAG:Ce” phosphor), that “down-converts” the wavelength of some of the blue light emitted by the LED, changing its color to yellow. In a blue LED/yellow phosphor lamp, the blue LED produces an emission with a dominant wavelength of, for example, about 450-460 nanometers, and the phosphor produces yellow fluorescence with a peak wavelength of, for example, about 550 nanometers in response to the blue emission. Some of the blue light passes through the phosphor (and/or between the phosphor particles) without being down-converted, while a substantial portion of the light is absorbed by the phosphor, which becomes excited and emits light across a broad spectrum that has a peak wavelength in the yellow color range (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light may appear white to an observer. Such light is typically perceived as being cool white in color. In another approach, light from a violet or ultraviolet emitting LED may be converted to white light by surrounding the LED with multicolor phosphors or dyes. In either case, red-emitting phosphor particles may also be added to improve the color rendering properties of the light, i.e., to make the light appear more “warm,” particularly when the single color LED emits blue or ultraviolet light.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights, specialty lighting applications and even as replacements for conventional incandescent and/or fluorescent lighting in general lighting and illumination applications. In many of these applications, it may be desirable to use luminescent materials to provide a lighting source that generates light having specific properties.